I live in London and own a 40d M Sport. During summer my car has 19s (prefer the ride quality to 20s) and for winter I went for the wheels recommended by BMW Europe, the cross spoke style 177 18in with Continental Winter Contacts. I thought about 19s from a visual perspective but went for the 18s as they're better in the snow / ice. It's personal taste but I like how they look, and driving on snow / ice they were brilliant.
I still got stuck in a car park in the Swiss Alps following a very heavy snowfall, so personally advise against anything larger if you expect to encounter extreme conditions...
